Lamb Growth Rate On Pasture : Effect Of Grazing Management, Sward Type And Supplementation

Abstract

In spring-lambing flocks an important objective is to achieve high lamb growth rate on pasture so that most lambs are drafted for slaughter by September. Lamb growth rate can vary greatly depending on the type of pasture being grazed. A series of grazing trials was carried out to assess the effect of pasture type, sward height, herbage allowance and concentrate supplementation on lamb growth rate pre and post weaning. Sward height was a useful indicator of the suitability of pasture for sheep grazing. A height of about 6 cm was near optimum for set stocking until late May. A decline in lamb growth rate frequently experienced in the month pre-weaning in June can largely be prevented if sward height is increased to 6 to 8 cm, or the flock grazed on aftergrass at a similar height. With rotational grazing, tight grazing to a residual sward height of 4 cm was beneficial in preventing the pasture becoming stemmy in June but reduced lamb weaning weight. Tight grazing in April when pasture is leafy is less restrictive on lamb growth than in June when the base of the sward is more stemmy. Post grazing heights of 4, 5 and 6 cm for April, May and June respectively, are suggested as a guide in order to achieve high lamb growth. There was a response to creep-feeding lambs. When concentrates were offered at 250 g/lamb/day from age 5 to 14 weeks the extra liveweight at weaning was associated with feed conversion ratios of 4.4 to 6.3 Creep grazing increased lamb weaning weight by over 2 kg and facilitated grazing the pastures tightly in June without penalising lamb growth. Lamb growth to weaning was better on pasture not grazed by sheep in the previous year and this benefit was at least partly attributable to lower level of parasites on the pasture. Lamb growth rate on pasture post-weaning varied greatly, from under 100 to over 200 g/day depending on the type of pasture grazed. For set stocking a sward height of 8 to 9 cm was required to maximise lamb growth. For rotational grazing, swards should be grazed down to about 6 cm. However the effect of sward height is modified by previous grazing management, in that tight grazing pre-weaning results in a more leafy pasture and higher lamb growth at comparable sward heights post-weaning. Pasture type also affected lamb growth. There was little difference between old permanent pasture and a mainly perennial ryegrass pasture when grazed at similar sward heights, or when lambs were given similar herbage allowances. However growth rates were considerably higher on grass/clover swards at equivalent allowances or similar sward heights. There was a close relationship between herbage allowance and lamb growth, with highest growth rate achieved at an allowance of about 5 kg of dry matter per lamb per day. Concentrate supplementation of weaned lambs on pasture (at 250 to 550 g/lamb/day) increased liveweight, carcass weight and kill-out proportion. Response to concentrates was slightly better on short grass. Feed conversion ratio for carcass gain ranged from 7 to 12 on short grass, 7 to 20 on long grass and was 14 when concentrates were offered ad libitum. Concentrate supplementation resulted in a higher proportion of lambs being drafted off pasture by late September (90 to 100 %) compared with 60 to 65% for lambs on grass only.